Does Running Burn Fat or Muscle?

The two primary forms of energy when you run are carbohydrates and fat. When you first start out on a run, the body immediately begins burning up carbohydrates, and as you continue to run, it switches over and starts burning fat. Both types of fuel are found in your blood as well as within your muscles. The muscle itself isn’t burned up, but instead is the primary tissue in the body that oxidizes, or burns, fat and carbohydrate for energy.

  1. Muscles and Fat

    • One energy source for running is the fat within your muscles. Even a muscle that appears to be quite lean contains droplets of triglyceride fat, and these droplets give the working muscle the energy it needs to carry on the steady and extended contractions you’re asking for as you run. Carbohydrates are the primary energy source during the first 20 to 30 minutes of your run, but as supplies run low, the body switches to fat for energy.

    Amino Acids Chip In

    • Amino acids from protein are also used for the oxidative metabolism needed to fuel the contractions of running muscles. Protein typically becomes a source of energy during long runs when the muscle’s glycogen stores become depleted. When this happens, the amino acids leucine, isoleucine and valine can contribute from 10 percent to 15 percent of your energy needs.

    Hormone Helps Fat Burning

    • During an exercise like running, muscles release a hormone called irisin, which turns white fat cells, which store fat, into brown fat cells, which burn fat. These brown fat cells continue burning after after you’re done exercising. Consequently, researchers are looking at irisin as a possible treatment to prevent or overcome insulin resistance, which leads to type 2 diabetes.

    Fit Runners and Fat

    • The more fit a runner is, the more she will burn fat during a run. Most people will burn more fat at low to moderate exercise intensities -- of 50 to 70 percent of their maximum heart rate -- because at that level, they are getting the extra oxygen they need to oxidize fat. As they increase the intensity to higher than 70 percent, however, their bodies have less oxygen to work with, so their bodies switch over to faster-burning carbohydrates. But fit runners have trained their bodies to use oxygen more efficiently, and as a result, they can burn fat at all intensity levels.

    Offsetting Muscle Loss

    • Muscle does not turn to fat, but everyone naturally starts losing muscle mass after the age of 25. Exercise offsets that natural loss. Consequently, the Centers for Disease Control and Prevention not only recommends regular aerobic exercise like running -- or some other workout like swimming or cycling that rhythmically utilizes large muscle groups in a way that elevates a person’s heart rate -- it also recommends a regular regimen of muscle-strengthening exercises for the legs, hips, back, abdomen, chest, shoulders, and arms.